A reverse osmosis (RO) membrane filtering device is used, for instance, to desalinate seawater or to produce purified water. There is a type of RO membrane filtering device that includes a plurality of spiral RO membrane elements accommodated in a pressure container. A spiral RO membrane element includes a permeate-water channel and an RO membrane disposed around the permeate-water channel. If more than one spiral RO membrane elements are to be disposed in series, permeate-water channels are connected via plugs.
With a large number of RO membrane elements disposed in series, there may be considerable variation in the amount of permeate water between RO membrane elements in the front and RO membranes in the rear.
In view of this, Patent Document 1 discloses a spiral seawater desalinating device with a permeate-water channel closed in the middle with a plug to be divided into a front permeate-water channel and a rear permeate-water channel, and a front permeate-water amount obtained by the front permeate-water channel is adjusted by a pressure adjusting valve. With this configuration, the flow rate of the front permeate water is adjusted, and thereby it is possible to reduce variation in the amount of permeate water among a plurality of RO membrane elements connected in series.